HEAT grill to medium-high heat.
BRUSH bread slices with 1 Tbsp. dressing. Brush chicken with 1 Tbsp. of the remaining dressing.
GRILL chicken 5 min. on each side or until done (165°F). Meanwhile, grill bread slices 1 min. or until lightly toasted on both sides, turning over after 30 sec.
COMBINE tomatoes, cheese and basil. Toss spinach with onions and remaining dressing; spoon onto serving plate. Top with chicken and tomato mixture. Serve with bread.